1. Field of the Invention
The invention relates to liquid-solid separation. More specifically, the invention relates to recovering useable water and solids from salt and brackish water and water-based solutions.
2. Discussion of Related Art
Many techniques have been developed for separating liquids from solids in water-based compounds, such as where particles and substances are dissolved in or suspended in water. Examples of such compounds include solid and particulate matter dissolved mixed, or in suspension in industrial and urban polluted waters, which may contain dissolved metals, dissolved complex organics, solvents and emulsions, radioactive contaminants and others. Other compounds include naturally occurring sea and brackish waters, mineralized waters, or man-made solutions used in industrial processes, food processing, fossil fuels extraction, mineral extraction and others. The present invention is suited for solid-liquid separation of all the above, with very low capital and operating costs.
Current separation methods use a variety of techniques for separating solids from their liquid bases. In general, the techniques focus on recovery of the solid component, neglecting the liquid. With respect to saline and contaminated waters, applied techniques are purposed at rejecting solid particulate and recovering the water base. In this latter case however, present inefficient separation methods result in poor recovery and costly processing, such as those that involve subjecting solutions to chemical treatments and heating; evaporating the liquids through boiling of the solutions and recovering some of the liquid through condensation; forcing the liquids through high pressure devices; and/or passing them through special membranes to retain some or all of the molecules of the dissolved particles.
Current techniques for water production and treatment are numerous. However,--commercially implemented methods that are able to obtain fresh water from saline and/or contaminated waters are few, the main ones being distillation, reverse osmosis and electrodyalisis. Although substantially different in approach, each technique is fundamental flawed for efficient water recovery by being very energy-intensive. This, in turn, causes high capital and operating costs. Additionally, performances associated with each technique tend to be very low: 25% to 40% for distillation; 30% to 50% for reverse osmosis; and lower still for electrodyalisis.
Most current water treatment systems for producing sizeable amounts of potable water require substantial amounts of energy in the form of heat and high pressures. As a result, the systems require expensive process equipment such as pressure vessels, heat exchangers and chemical digesters and processors. Major water treatment systems also involve filtration, which requires the use of expensive, perishable filtering organic membranes, thus are fettered by high capital and operating costs, which results in uneconomical recovery expenses.
This high-energy dependency also tends to result in significantly low performance. If energy is equivalent of `work`, the amount of work applied to a given separation process is geometrically proportional to the amount of solids dissolved in the solutions. A solution having high-solid contents requires more work to separate the solids from the solution than a solution that has less solid contents. The more efficient processes, in the best of circumstances, require at least 50 joules per gram of solution treated, which far exceeds the 2.5 joules per gram theoretically needed for separating solids from its water base. This excess work lowers performances and substantially increases process costs.
Some processes require high-temperature environments, gaseous high-speed currents or compressed air to effect the separation. Some processes involve drying by pulverizing heated solutions; atomizing hot liquids; drying in fluid beds; filtering through membranes; atomizing compressed air-liquid mixtures, etc. However, each process for solid-liquid separation application has serious limitations, especially in the separation of suspended or dissolved particles in water solutions. A major limitation is the requirement of an average of 2,000 joules per gram of solution treated, mostly in the form of heat, electricity, high pressures or a combination of the three.
In the separation of salts in sea and brackish waters, the use of compressed air to drive and atomize the saline solutions transfers the inefficiencies of a low performance energy-intensive driver, such as compressed air, resulting in operating costs equal or greater than other conventional desalination methods, such as distillation. Moreover, as compressed air disperses and diffuses the water vapor more than any other medium, the large masses of air mixed with the vapor require large condensing cooling devices, resulting in higher capital costs.
Spanish Patent No. ES 2,018,732, issued May 1, 1991, to M. Lumbreras y Gimenez; and U.S. Pat. No. 5,207,928, issued May 4, 1994, to E. J. Lemer describe generating, with compressed air, a stream of high-velocity saltwater droplets that vaporize without being heated. Salt precipitates from the vaporizing liquid and is recovered in a pan while the resulting water vapor is recovered by showering the water vapor with liquid water. Saltwater is mixed with compressed air. This mixture then is directed through an indistinct pneumatic nozzle that atomizes the mixture in a chamber where temperature and relative humidity are at ambient (room) levels. The volume and effect of compressed air mixed with the water and the high velocity of the mixture at the nozzle exit not only limits the volume of water that can be recovered, but diffuses the vapor inside the chamber by an entrained air mass that is approximately 30 times larger at a short distance from the nozzle's orifice. Diffusing water vapor into the chamber supersaturates the ambient air. At a relative humidity of 100% or more, air is unable to provide the energy necessary for evaporation, which impedes the process. Also, large amounts of air induce the diffused vapor to recombine with the separated salt particles.
U.S. Pat. No. 4, 323,424, issued Apr. 6, 1982, to D. J. Secunda et al. also addresses desalinization. However, both the '424 and the '928 patents do not address the formation of micronsized, non-evaporated droplets that are indistinguishable from vapor. This reduces the amount of fresh water produced. Fresh water production is further complicated by the minute size of the droplets, typically 1 to 10 microns in diameter. Once the droplets evaporate, the resulting salt particles are of sub-micron size. For example, a saline droplet of 1 micron in diameter, with an average NaCl content of 3.5% by weight, as it is the case with seawater, would precipitate a particle less than 1/50.sup.th of one micron in diameter. According to Stokes' Velocity of Sedimentation Law, particles up to 1 micron in diameter tend to behave as molecules and remain suspended in the air for indefinite periods, thus are able to recombine with the water vapor. For brackish waters with salt content of 0.5% by weight or less, processing is very difficult, as the solid salt particles would have diameters smaller than 0.005 microns. Only droplets of 30 microns in diameter and up will shed salt particles large enough to fall quickly by gravity. Thus, far from dropping to the bottom of the chamber as these patents describe, salt particles derived from droplets less than 30 microns in diameter will remain suspended in the air for an undetermined amount of time, recombine with the water vapor produced and be transported with the vapor to the recovery chamber, driven by blower-produced air currents. Thus, the collected liquid will be mostly saltwater.
Additionally, a substantial amount of vapor is produced by the small water droplets that readily mixes with the large masses of ambient (secondary) air entrained by the initial compressed (primary) air at nozzle exit. This rapidly spreading vapor not only cannot change into water mist, since the large masses of entrained air impede condensation, but the vapor also fills the chamber quickly. A chamber filled with vapor causes an entropy dilemma, whereby the increase in overall humidity levels saturates the chamber, exhausting the potential for evaporation of the secondary air. This vapor will warm up and expand. This in turn disables the remaining air from vaporizing additional droplets. Without heat from the secondary air, the separation process stops.
Some of the techniques described counter these adverse effects by generating an ambient air current in an upward and oblique direction with a fan or blower at successive intervals from the lower portion of the chamber. However, the air current crossing the path of the suspended sub-micron salt particles fuses the salt particles with expanded fresh water vapor. Further, the intermittent blower also tends to recombine a portion of the separated water with larger salt particles, falling to the bottom of the chamber when the blower is off. The blower equipment also increases capital and operating expenses, thus increasing the final cost of any suitable water collected.
Although these techniques support a low air-water ratio of 1:10, the use of a compressor consumes over 18 kW/h per cubic meter of fresh water produced. This high power consumption renders the process non-competitive with current desalination techniques, such as distillation, which uses less power.
In view of the above, producing inexpensive, reusable water requires a separation process that derives a product with the least possible work. Economically obtaining reusable water from contaminated or sea/brackish waters ideally should separate the contaminants and salts from their liquid bases at normalized or standard room temperature and pressure (STP). A water recovery process that operates at STP eliminates the need for energy-intensive process engineering that otherwise would be needed to drive the separation. Unfortunately, none of the foregoing provides a method and apparatus for economical solid-liquid separation in water base solutions that separates contaminants and salts from their liquid bases at standard room temperature and pressure. None of the aforementioned references, taken alone or in combination, are seen as teaching or suggesting the presently claimed Method and Apparatus for Economical Solid-Liquid Separation in Water-Based Solutions.